Vertebral Osteomyelitis Caused by Mycobacterium arupense Mimicking Tuberculous Spondylitis: First Reported Case and Literature Review

Abstract Mycobacterium arupense is a slow-growing, nontuberculous mycobacterium widely found in the environment and is known to cause tenosynovitis and osteomyelitis, mainly in the hands and wrists. We present the first case of vertebral osteomyelitis caused by M arupense in a 78-year-old man with renal cell carcinoma. The patient had a history of tuberculous pleuritis in childhood. Although the nucleic acid amplification test of the vertebral tissue for Mycobacterium tuberculosis was negative, we initiated tuberculosis treatment based on the history and pathological findings of auramine-rhodamine-positive organisms and epithelioid cell granulomas. Subsequently, the isolated mycobacterium was identified as M arupense by genome sequencing. Accordingly, the treatment regimen was changed to a combination of clarithromycin, ethambutol, and rifabutin. Owing to a subsequent adverse event, rifabutin was switched to faropenem, and the patient was treated for a total of 1 year. In previous literature, we found 15 reported cases of bone and soft tissue infections caused by M arupense, but none of them had vertebral lesions. Physicians should be aware that M arupense can cause vertebral osteomyelitis mimicking tuberculous spondylitis. In addition, molecular testing of isolated mycobacteria is essential for diagnosis, even if tuberculous spondylitis is suspected.

Mycobacterium arupense is a slow-growing nontuberculous mycobacterium (NTM) that was first isolated from clinical specimens in 2006 [1]. Since then, only a few cases of bone and soft tissue infections caused by M arupense have been reported. Here, we report the first case of vertebral osteomyelitis caused by M arupense, along with a literature review of bone and soft tissue infections caused by this pathogen.

CASE REPORT
A 78-year-old Japanese man presented with lower back pain of 8 months' duration and bilateral leg pain of 1 month's duration. He had developed tuberculous pleuritis at 11 years of age. The patient had no history of trauma. His mother had a history of tuberculosis when he was 7 years old. He had no occupational or avocational exposure to wet soil or water. He had never traveled abroad.
At the presentation, the patient's vital signs were normal. Physical examination revealed an increased patellar tendon reflex on the left side and bilaterally decreased Achilles tendon reflexes. He was capable of bending both knees and moving both ankles as intended. Blood tests showed normal white blood cell counts and elevated C-reactive protein level (6.6 mg/dL) and erythrocyte sedimentation rate (55 mm/hour). Mild renal dysfunction was observed, with a creatinine of 1.35 mg/dL. The soluble interleukin-2 receptor level was elevated to 764 U/mL. T-SPOT.TB (Oxford Immunotec) and anti-human immunodeficiency virus antigens/antibodies tested negative. Computed tomography revealed a fracture with osteolytic changes in Th12-L1 vertebrae and a mass in the right kidney ( Figure 1). Magnetic resonance imaging revealed Th12-L1 vertebral fractures and protrusion of the vertebrae into the spinal canal ( Figure 2). Three days after presentation, he underwent posterior vertebral decompression and percutaneous thoracolumbar root screw fixation for neurological deficits and vertebral biopsy for a pathological vertebral fracture. No remarkable abscess or necrotic tissue was observed intraoperatively. Vertebral tissue samples were negative for general bacterial culture, concentrated fluorochrome smear microscopy, and nucleic acid amplification testing for Mycobacterium tuberculosis. Pathological examination of the vertebral tissue showed caseous necrosis surrounded by epithelial granulomas and a few multinucleated Langhans giant cells (Figure 3). A few acid-fast bacilli were found on auramine-rhodamine and acid-fast stains;  no neoplastic lesions were detected ( Figure 4). Based on these findings, the patient was presumed to have tuberculous spondylitis, and oral rifampicin, isoniazid, ethambutol (EMB), and pyrazinamide administration was commenced on the eighth postoperative day.
Bacteriological investigation revealed the growth from vertebral tissue cultures on 2% Ogawa medium (Kyokuto Pharmaceuticals) in the second week at 30°C and in the third week at 37°C, respectively. Cultures on a liquid medium (Mycobacteria Growth Indicator Tube, Becton Dickinson) at 37°C were negative for 12 weeks. Bacteria and mycobacteria were not isolated from sputum or urine cultures. Mycobacterium tuberculosis group antigen or mycobacterial protein fraction from bacille Calmette-Guérin of Rm 0.64 in electrophoresis tested negative. Matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry indicated an NTM suggestive of M arupense infection. Based on these results, pyrazinamide was switched to clarithromycin (CLR) on postoperative day 27. Subsequent identification tests revealed 100% homology with M arupense on 16S ribosomal RNA (rRNA), 99.75% on hsp65, and 98.91% on rpoB gene sequencing. The relationship between 16S rRNA gene sequence of this isolate and that of other species of Mycobacterium terrae complex (MTC) is shown in the Supplementary Figure. Drug susceptibility testing showed that the isolate was susceptible to rifabutin (RFB), EMB, and CLR based on the interpretation for Mycobacterium kansasii of the Clinical and Laboratory Standards Institute document M62 (Table 1) [2]. The regimen was then adjusted to 3-drug combination therapy of RFB, CLR, and EMB on postoperative day 126. Subsequently, neutropenia and thrombocytopenia, likely due to the interaction between RFB and CLR, were observed, and RFB was switched to faropenem (FRPM) on a postoperative day 160. Antimicrobial therapy was completed 1 year after the initiation of CLR treatment. His symptoms gradually improved, and his condition has been stable for 2 years after treatment. A partial nephrectomy was performed for renal cancer.

DISCUSSION AND LITERATURE REVIEW
To the best of our knowledge, this is the first reported case of vertebral osteomyelitis caused by M arupense infection.   The organism was first isolated from clinical specimens by Cloud et al in 2006 [1] and was classified as MTC, which are slow-growing NTM. Recent developments in genetic analysis have revealed that M arupense, like other bacteria of the MTC, is widely distributed in the environment, including in soil and surface water [3,4]. MTC causes bone and soft tissue infections through direct exposure to environmental organisms [5]. Nevertheless, M arupense is a rare pathogen causing NTM infection in humans. We reviewed the English and Japanese literature on clinical cases of bone and soft tissue infections caused by this organism using PubMed, Google Scholar, and Ichushi-Web (a bibliographic database of articles published in Japanese-language medical journals). The search term was "Mycobacterium arupense." Fifteen cases were identified, 13 of which were tenosynovitis or osteomyelitis of the fingers, hands, or wrists ( Table 2) [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. In addition to other clinical manifestations, pneumonitis and bacteremia have been reported [21,22]. No previous cases of M arupense infection presenting with vertebral osteomyelitis have been reported. Additionally, this organism has not been isolated or identified from the vertebrae in retrospective molecular studies of clinical specimens [10,[23][24][25].
The patient was initially suspected of having tuberculous spondylitis based on the pathological findings of granuloma with caseous necrosis, auramine-rhodamine stain-positive organisms, and a family and personal history of tuberculosis. Generally, NTM is unlikely to be at the top of the differential list because of its rarity among the causative organisms of vertebral osteomyelitis [26]. Furthermore, the acid-fast bacilli smear test and culture of biopsy specimens cannot differentiate NTM from M tuberculosis. This makes molecular identification methods crucial when mycobacteria are detected in vertebral specimens, even when M tuberculosis is suspected. However, careful interpretation is needed to determine if the M arupense detected is the etiological cause, given the clinical presentation and the possibility of contamination. The pathogenicity of M arupense detected in respiratory specimens is unknown [27], especially in patients with cancer. A retrospective observational study reported comparable outcomes with and without antimicrobial therapy in patients with cancer in whom M arupense was isolated from respiratory specimens, indicating colonization [25]. We diagnosed this case as vertebral osteomyelitis caused by M arupense because it was detected in a vertebral lesion, and there was no strong evidence of a differential diagnosis, such as metastatic malignancy or tuberculosis.
Bone and soft tissue infections with M arupense, as with other NTM, predominately occur in patients with immunocompromised conditions (eg, immunosuppressive drug use and diabetes mellitus), history of injury, and activities that increase the risk of hand injury and environmental exposure (eg, farming and fishing) ( Table 2). In contrast, NTM vertebral osteomyelitis is rarely caused by direct inoculation from an injury. This phenomenon is more commonly observed in immunocompromised individuals [28]. Our patient had no history of injury; however, he was found to have renal cancer, which may have been a predisposing factor. Little is known about the pathophysiology of NTM vertebral osteomyelitis in patients without a history of trauma. However, it has been hypothesized that NTM that colonize the respiratory or gastrointestinal tract mucosa are taken up by endocytosis when they come into contact with macrophages, which then mobilize to the site of bone formation and release the NTM, leading to localized osteomyelitis [26].
The optimal antimicrobial regimen and treatment duration for M arupense has not yet been established. There are no randomized controlled trials or comparative studies evaluating the management of osteomyelitis due to NTM; therefore, management relies on case reports, reviews of cases, drug susceptibility testing results, and official American Thoracic Society/Infectious Diseases Society of America statements [29]. However, combination antimicrobial therapy, often combined with surgical debridement and drainage of abscess, is commonly performed for localized vertebral osteomyelitis since it can serve as a storage site for NTM [26,30]. In our patient, these surgical procedures were not performed because no abscess or necrotic tissue was observed during the decompression. We initiated a standard 4-drug combination therapy for tuberculosis because we initially considered a diagnosis of tuberculous spondylitis based on the histological findings and family history. Previous studies on the in vitro susceptibility of M arupense to antimicrobial agents have shown that it is often susceptible to amikacin, CLR, EMB, RFB, and sulfamethoxazole-trimethoprim but resistant to ciprofloxacin, levofloxacin, and rifampicin [5,10,23]. Thus, standard 4-drug therapy targeting M tuberculosis may be inappropriate. In addition, even if an isolate is susceptible to a particular drug in vitro, careful follow-up is crucial to ensure a clinical response. We treated our patient with CLR, EMB, and RFB after confirming that the strain was sensitive to these 3 drugs, which is consistent with many previous reports. Subsequently, we switched from RFB to FRPM due to the appearance of side effects; no previous cases of bone and soft tissue infections due to this pathogen were treated with FRPM (Table 2). Furthermore, although the effectiveness of FRPM against M tuberculosis and rapidly growing mycobacteria has been suggested in case reports [31][32][33], its effectiveness against slow-growing NTM has rarely been described. Given the susceptibility results and the successful clinical course of this case after treatment, FRPM may be considered an alternative drug candidate. Vertebral osteomyelitis due to NTM requires antimicrobial therapy for at least 4-6 months after therapeutic response. The duration may be considered a year or more in immunocompromised individuals or in those with inadequate debridement [26]. Previous cases of bone and soft tissue infections caused by M arupense were treated for approximately 1 year regardless of the immune status, and improvements were observed ( Table 2). In accordance with these findings, we treated the patient for 1 year.
Bone and soft tissue infections caused by M arupense have a favorable prognosis in most cases (Table 2). Our patient also improved and had no recurrence.

CONCLUSIONS
Herein, we presented the first case of vertebral osteomyelitis caused by M arupense in a patient with renal cancer. He improved after 1 year of antimicrobial therapy with no relapse. M arupense can cause vertebral osteomyelitis mimicking tuberculous spondylitis; therefore, greater emphasis should be placed on pathogen recovery and definitive molecular testing for the mycobacterium in the affected tissue.

Supplementary Data
Supplementary materials are available at Open Forum Infectious Diseases online. Consisting of data provided by the authors to benefit the reader, the posted materials are not copyedited and are the sole responsibility of the authors, so questions or comments should be addressed to the corresponding author.

Notes
Author contributions. Conception of the work, data collection, and literature review: A. K., K. F. Drafting the first manuscript: A. K. Critical revision of the manuscript: K. F., S. M., Y. I., Y. N., M. H., N. S. All authors approved the submitted manuscript version and have agreed to be personally accountable for any questions related to the accuracy or integrity of any part of the work.